In the present applicants' U.S. Pat. No. 4,796,573 dated Jan. 10, 1989, the hydraulic valve lifter includes a lash adjustment mechanism in which a lash adjusting piston defines cooperatively with a valve damping piston, a lash adjusting chamber that is communicated through a check valve carried by the latter piston directly to a pressure chamber which is cooperatively defined by the valve damping piston and a cam follower piston and which is supplied with pressurized hydraulic fluid in the form of oil from the engine's oil system. Further development work on the lash adjusting mechanism has revealed that the magnitude of oil pressure that acts on it can influence its performance. Specifically, it has been found that a reduction in the oil pressure magnitude acting on the lash adjusting mechanism can improve the lash adjustment function. The problem is therefore posed as to how to create such a pressure reduction with minimum revision of existing hardware and/or addition of new hardware, and without attenuating hydraulic pressures in locations where such attenuated pressures would be unacceptable.
The present invention provides an ingenious solution to this problem. Pressure attenuation is achieved only for the hydraulic fluid supplied to the lash adjusting mechanism so that hydraulic pressures at other locations do not have to be attenuated.
The hydraulic pressure attenuation at the lash adjusting mechanism is accomplished by modifying the hydraulic fluid communication path between the aforementioned pressure chamber and lash adjusting chamber to include a restriction that is formed by the sliding clearance between the lash adjusting piston and the valve damping piston. A circular annular groove extends around the outside of the damping piston and is in direct communication with the termination of the sliding clearance restriction. A slant hole extends radially inwardly from the circular annular groove and ends at an intersection with a central axial blind hole that is open to the lash adjusting chamber. The sliding clearance restriction creates a pressure drop such that the pressure of hydraulic fluid in the circular annular groove is significantly less than the hydraulic pressure in the pressure chamber, and this reduced pressure is delivered through the slant and axial holes in the damping piston, which themselves may supply some small, but relatively insignificant, additional pressure drop.
The relative dimensions of the O.D. of the lash adjusting piston and the I.D. of the valve damping piston can be controlled accurately enough by conventional manufacturing processes such that a desired pressure attenuation in the sliding clearance restriction results. The circular annular groove provides a suitable surface with which a drill bit of sufficient strength can be engaged for drilling the slant hole. Thus, the invention enables the pressure reduction for the lash adjusting piston to be embodied in the lifter solely by conventional machining operations and without any additional parts beyond those of the lifter of U.S. Pat. No. 4,796,573.
The foregoing, as well as additional, features, advantages, and benefits of the invention, will be seen in the ensuing disclosure which includes a drawing of the best mode contemplated at the present time in carrying out the invention.